Process for the reduction of copper oxide

ABSTRACT

The present invention provides a method of reducing copper oxide to improve its acid resistance comprising the step of reacting cupric oxide with a reducing agent selected from the group consisting of morpholine borane, ethylene diamine borane, ethylene diamine bisborane, t-butylamine borane, piperazine borane, imidazole borane, and methoxyethylamine borane to form cuprous oxide.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for the reduction ofcopper oxide, and, particularly, to a method for reducing copper oxideto improve acid resistance.

BACKGROUND OF THE INVENTION

[0002] A variety of methods have been investigated concerning thesurface treatment of copper before bonding a resin to the copper. Undersome methods, the surface of a copper printed circuit was chemicallytreated or polished to form a suitable extent of microscopicirregularities as a pretreatment prior to solder resist printing toimprove the bonding strength between the solder resist and the coppercircuit. In such treatments, however, there was a problem of coppermigration generating electrolytic corrosion deposits, often in the formof dendrites, which results in reduction of the intracircuit spacing.This problem, in turn, led to reduced intracircuit insulationresistance.

[0003] Because sufficient bonding strength between a metal and a resingenerally cannot be secured by directly bonding the resin onto thesmooth and even surface of the metal, an oxide layer is often formed onthe surface of the metal to improve the bonding strength. Accordingly,investigations have been made on a copper surface treatment method inwhich an oxide layer of cupric oxide (CuO), cuprous oxide (Cu₂O), or thelike is formed on the surface of copper to improve its bonding strength.Unfortunately, some metallic oxides are readily hydrolyzed upon contactwith an aqueous acidic solution and become dissolved therein as metallicions.

[0004] A number of studies have shown, however, that acid resistance isimproved by first forming cupric oxide on the surface of the copper andsubsequently reducing the cupric oxide to cuprous oxide. In general,cuprous oxide is less soluble in an acid than cupric oxide. A number ofmethods of reducing cupric oxide to cuprous oxide have been developed.In U.S. Pat. No. 4,642,161, for example, a solution containing at leastone reducing agent represented by the general formula BH₃NHRR′, whereinR and R″ are each selected from the group consisting of H, CH₃ andCH₂CH₃, is used to reduce cupric oxide. The most commonly practicedmethod of reducing cupric oxide to form cuprous oxide, however, is byuse of the reducing agent dimethylamine borane (DMAB), which has theformula (CH₃)₂NH.BH₃. See Japanese Patent Application No. 1 219,483(1989).

[0005] Unfortunately, current methods for the reduction of cupric oxideto form cuprous oxide suffer from a number of significant drawbacks. Ingeneral, DMAB and other reducing agents for such a reaction exhibit poorthermal stability and decompose rapidly whenever their temperaturereaches a level of approximately 40° C. to 50° C. Moreover, thesecompounds are often very noxious. Further, the pH for reductionreactions with such compounds typically must be maintained in thelimited range of 12 to 13 to prevent hydrolysis of the reducing agent.

[0006] It is, therefore, very desirable to develop a method of reducingcupric oxide to form cuprous oxide which reduces or eliminates these andother drawbacks associated with current methods.

SUMMARY OF THE INVENTION

[0007] In general, the present invention provides a method of treatingcopper oxide comprising the step of reacting the cupric oxide with areducing agent selected from the group consisting of morpholine borane(C₄H₁₂BNO), ethylene diamine borane (H₂NCH₂CH₂NH₂.BH₃), ethylene diaminebisborane (H₂NCH₂CH₂NH₂.2BH₃), t-butylamine borane ((CH₃)₃CNH₂.BH₃),piperazine borane (C₄H₁₀N₂.BH₃), imidazole borane (C₃H₄N₂.BH₃) andmethoxyethylamine borane (C₃H₉NO.BH₃). Most preferably, the reducingagent is morpholine borane.

[0008] The reducing agents of the present invention have been discoveredto provide a number of significant advantages over the various reducingagents currently used in the reduction of cupric oxide to cuprous oxide,and particularly over DMAB. For example, the thermal stabilities of thereducing agents of the present invention are much better than currentreducing agents. The reducing agents of the present invention can alsoreact at a very broad temperature range without thermal decomposition.Preferably, the reaction temperature is in the range of approximately 3°C. to approximately 95° C. Reaction at elevated temperatures using thereducing agents of the present invention may decrease the process timeover current procedures.

[0009] Moreover, reducing agents of the present invention are relativelyodor free, making handling thereof less noxious. Likewise, the pH valueof the aqueous solutions of the reducing agents of the present inventioncan range from approximately 4 to approximately 14. Preferably, the pHranges from approximately 7 to approximately 13. The concentration rangeof the reducing agent in the reaction solution is preferably in therange of 0.1 wt % to the solubility limit of the reducing agent in thesolvent. More preferably, the concentration range of the reducing agentin the reaction solution is in the range of 0.3 wt % to the solubilitylimit of the reducing agent in the solvent. As used herein, wt % isdefined as follows:$\frac{{weight}\quad {of}\quad {reducing}\quad {agent}}{( {{{weight}\quad {of}\quad {reducing}\quad {agent}} + {{weight}\quad {of}\quad {solvent}}} )} \times 100\quad \%$

DETAILED DESCRIPTION OF THE INVENTION

[0010] Copper oxide (cupric oxide) was formed on copper test coupons viawell known methods described in the literature or through the use ofcommercial pre-formulated solutions. The weight gain of the oxidecoating was typically in the range of approximately 0.30 to 0.50 mg/cm².The process cycle included an alkaline soak, water rinse,neutralization, water rinse, sodium hydroxide treatment, and copperoxide treatment. The copper oxide immersion time ranged from 4 to 6minutes at approximately 160 to 170° F.

[0011] Aqueous solutions of the reducing agents of the present inventionand deionized water were prepared in a concentration ranging fromapproximately 1 g/L (1 gram/liter of solution or 0.1 wt %) and higher.Preferably, the concentration of reducing agent was in the range ofapproximately 0.1 wt % to the solubility limit of the reducing agent. Inthe case of morpholine borane, the concentration of the reducing agentis more preferably in the range of approximately 1 to approximately 20gm/L (that is, approximately 0.1 wt % to approximately 2.0 wt %).

[0012] Preferably, the pH of the solution is in the range ofapproximately 5 to approximately 13. More preferably, the pH of thesolution is in the range of approximately 7 to approximately 13. The pHof the solution was adjusted with sodium hydroxide. The reactiontemperature is preferably in the range of approximately 3° C. toapproximately 95° C. More preferably, the reaction temperature is in therange of approximately 20° C. to approximately 50° C. A number of proticsolvents other than water including alcohols (for example, methanol) aresuitable for the reactions of the present invention.

[0013] During the reduction reaction, cupric oxide coated test couponswere immersed in a reducing agent solution as described above. Vigorousbubbling occurred rapidly upon contact with the reducing agent solution.The test coupons were typically immersed for approximately 3 to 10minutes. The coupon surface was observed to change color from the“black” oxide coating to a uniform brown appearance, indicating thereduction of cupric oxide to cuprous oxide. Subsequent to the reducingreaction, the coupons were rinsed and immersed in 3 molar HCL. The colorof the coupon remained the same, indicating it was completely resistantto the aqueous acid solution. A summary of the results of severalexperiments performed with morpholine borane over a range of conditionsare set forth in Table 1 below. TABLE 1 Concentration of Acid MorpholineBorane Temperature Resistant (gm/L H₂O) (° C.) pH Coating 80 25 7 yes 8025 13 yes 80 32 7 yes 80 32 13 yes 10 25 7 yes 10 25 13 yes 10 32 7 yes10 32 13 yes 10 25 4 yes 10 50 4 yes 10 50 14 yes 10  4 7 yes 10 50 7yes 10 90 7 yes 10 24 4 yes 10 25 14 yes  3 25 7 yes

[0014] Table 2 below summarizes several studies performed on compoundsother than morpholine borane. TABLE 2 Conc. Temp. Acid Compound (wt %)(° C.) pH Resistance Appearance piperazine 1.0 23 7-8 Yes Medium boranebrown oxide ethylenediamine 1.0 24 7-8 Yes Medium diamine borane brownoxide ethylenediamine 0.75 24 7-8 Yes Dark brown bisborane oxideimidazole borane 1.0 24 7-8 Yes Dark brown oxide methoxyethylamine 1.023 7-8 Yes Medium borane brown oxide

[0015] Although the present invention has been described in detail inconnection with the above examples, it is to be understood that suchdetail is solely for that purpose and that variations can be made bythose skilled in the art without departing from the spirit of theinvention except as it may be limited by the following claims.

What is claimed is:
 1. A method of reducing copper oxide comprising thestep of reacting the copper oxide with a reducing agent selected fromthe group consisting of morpholine borane, ethylene diamine borane,ethylene diamine bisborane, t-butylamine borane, piperazine borane,imidazole borane, and methoxyethylamine borane.
 2. The method of claim 1wherein the reaction occurs in a solution of the reducing agent whereinthe concentration of the reducing agent is in the range of approximately0.1 wt % to the solubility limit of the reducing agent.
 3. The method ofclaim 2 wherein the concentration of the reducing agent is in the rangeof approximately 0.3 wt % to the solubility limit of the reducing agent.4. The method of claim 1 wherein the reaction occurs at temperature inthe range of approximately 3° C. to approximately 95° C.
 5. The methodof claim 4 wherein the reaction occurs at temperature in the range ofapproximately 20° C. to approximately 50° C.
 6. The method of claim 1wherein the solvent comprises water and the pH of the solution is in therange of approximately 4 to approximately
 14. 7. The method of claim 6wherein the pH of the solution is in the range of approximately 5 toapproximately
 13. 8. The method of claim 7 wherein the pH of thesolution is in the range of approximately 7 to approximately
 13. 9. Themethod of claim 2 wherein reducing agent is morpholine borane.
 10. Themethod of claim 9 wherein concentration of the morpholine borane is inthe range of approximately 0.1 wt % to 2.0 wt %.
 11. A method ofreducing cupric oxide to cuprous oxide comprising the step of reactingthe copper oxide with a reducing agent selected from the groupconsisting of morpholine borane, ethylene diamine borane, ethylenediamine bisborane, t-butylamine borane, piperazine borane, imidazoleborane, and methoxyethylamine borane.
 12. The method of claim 11 whereinthe reaction occurs in a solution of the reducing agent wherein theconcentration of the reducing agent is in the range of approximately 0.1wt % to the solubility limit of the reducing agent.
 13. The method ofclaim 12 wherein the concentration of the reducing agent is in the rangeof approximately 0.3 wt % to the solubility limit of the reducing agent.14. The method of claim 11 wherein the reaction occurs at temperature inthe range of approximately 3° C. to approximately 95° C.
 15. The methodof claim 14 wherein the reaction occurs at temperature in the range ofapproximately 20° C. to approximately 50° C.
 16. The method of claim 11wherein the solvent comprises water and the pH of the solution is in therange of approximately 4 to approximately
 14. 17. The method of claim 16wherein the pH of the solution is in the range of approximately 5 toapproximately
 13. 18. The method of claim 17 wherein the pH of thesolution is in the range of approximately 7 to approximately
 13. 19. Themethod of claim 12 wherein reducing agent is morpholine borane.
 20. Themethod of claim 19 wherein concentration of the morpholine borane is inthe range of approximately 0.1 wt % to 2.0 wt %.